Research Project V addresses the definition, classification and etiology of individual differences in comprehension and disabilities associated with reading comprehension deficits and oral language comprehension deficits. Project V proposes to continue administering extensive assessments of both listening and reading comprehension and passage fluency, across simple and complex discourse, and across ages 8-16, to understand the genetic, cognitive, and developmental aspects of comprehension. Twins and siblings selected for deficits either in reading, comprehension, writing, and/or ADHD, as well as normal-range control twins will be tested. There are 3 prongs to our approach to understanding the functional architecture of comprehension and the nature and etiology of deficits. One involves multivariate analyses of individual differences in both reading comprehension and listening comprehension, across different age groups and across different comprehension tests, using multiple measures of cognitive skills and knowledge. This modeling will be compared across our broad range of comprehension tests so that we can address the problem of why there are currently such dramatic inconsistencies in diagnosis of comprehension deficits and how the problem might be rectified. A second prong to understanding the components of comprehension skill and the origin of deficits is to use behavioral genetic analyses to assess their etiology in terms of genetic and environmental influences. These analyses will also show the degree to which component comprehension skills and other learning learning disabilities such as ADHD share genetic influences, and thus the degree to which they represent distinct or common deficits. The third prong is to further define the phenotype by finding variables often ignored by current tests that could be influencing comprehension performance, such as the centrality of ideas and how prior knowledge of the passage topic influences comprehension and fluency. In collaboration with other CLDRC projects, Project V will assess whether similar skills and similar genes may be influencing comprehension and writing (Project II), assess molecular genetic linkage and association for deficits in comprehension (Project IV), and longitudinally assess stability of comprehension problems (Project VI).Project V provides research on the cognitive, developmental, and genetic aspects of language comprehension that underlie some of the most prevalent learning disabilities. It advances knowledge about both biological and behavioral mechanisms in reading comprehension, listening comprehension, and their co-occurrence with other learning disabilities such as ADHD and writing difficulties.